I'd take it back to when you replaced the hard disk - did you set master/slave on the new drive? The fact that it's chiming, booting to a grey screen suggests all is good - so I'd investigate your IDE devices to make sure the right master/slave combination is all set. Whilst you're in there, by all means reset the CPU card, RAM, graphics card - etc.

Can you get into Open Firmware at all (command + option + O + F at chime)

JB